This invention relates to field emission cathodes. Field emission cathodes employ a field emission source, typically a minute tungsten crystal having an extremely fine emitting tip. The crystal is mounted on a filament which also serves, if the crystal is heated, as a heating element for the cyrstal. (As used hereinafter, the term "tip" means the entire crystal as well as its tip). The filament further serves to receive a source of electrical potential for electrically exciting the tip.
A field emission cathode, whether heated or cold, may be used in a variety of electron beam systems and devices which require or favorably utilize a very small, extremely bright source of electrons, such as electron microscopes and electron lithography systems.
This invention pertains particularly to an improved mount for the field emitting tip of a field emission cathode. Although this invention may have application in a variety of electron beam devices and systems, its need is extreme in any application wherein axial compactness of the cathode or associated gun is of critical importance.
Such an application exists in the electron beam memory systems described in my referent copending applications, Ser. No. 825,219 and Ser. No. 895,199.
It is a primary object of the inventions described and claimed in each of those applications to provide an electron beam memory system having an electron gun capable of developing high enough electron probe current densities to permit no-develop recording and small enough probe sizes to permit ultra-high density, high rate recording, yet of such low mass and compactness as to make feasible rapid random accessing of any area on the system's recording medium.
In the context of said systems, an improved mount for the electron field emitter is needed. It is known to mount the tiny field emission tip of a field emission cathode on a hairpin shaped filament. The axis of the "hairpin" is aligned with the electron beam axis; the tip is secured, as by welding, to the bend of the hairpin. The hairpin is carefully mounted such that the emitting tip is on the electron beam axis. As noted, the supporting filament is energized with a heater current in those applications in which the field emission is to be aided by heating the tip.
Such conventional field emission cathodes require mechanical and electrical isolation of the field emitter from the other gun components. The end result is typically a field emitter assembly having significant axial depth.
In electron beam memory systems of the type described in my above-identified copending applications, Ser. No. 825,219 and Ser. No. 895,199, in order to achieve the necessary fast access times of the electron beam head used for recording, reading or verification, it is imperative that the mass of the system's electron gun be as small as possible. Since size of the gun inevitably translates into mass, it is critically important in these systems that every dimension of the electron gun be reduced as small as possible.